The invention is directed to a method for determining the thickness and growth rate of a layer of ice on structural component parts of means of transport, particularly aircraft such as airplanes, helicopters and wind-driven power stations or the like, and to a device for carrying out the method, in which the surface of a structural component part which is to be monitored and which is covered by an ice layer and/or disturbing layer is illuminated in such a way that the radiation passing through the ice layer and/or disturbing layer is measured and evaluated in different wavelength regions.
Methods and devices for carrying out the method which are known from the prior art measure the surface state of traffic routes with respect to black ice, icing and any present deicing agents and also detect ice formation and frost formation on fixed surfaces such as road surfaces in order to inform travelers of road conditions in the event of unforeseen weather situations. Icing of roadways and sudden formation of glaze ice do not present dangers only in road traffic; in rail traffic and air travel, also, iced structural component parts due to freezing water also pose a threat. Iced structural component parts on aircraft, for instance, particularly on structural component parts in the wing area which move relative to one another, lead to hindrance of air traffic and are a considerable threat to air safety; on the other hand, even a small ice formation on vulnerable structural component parts of aircraft can lead to a drastic change in the profile characteristics of the utilized structural component parts. While more and more flights can be carried out under the most adverse weather conditions thanks to modern navigation systems, e.g., radar guidance or direction-finding systems, there is also a rise in the potential risk of icing of aircraft. Particularly smaller aircraft with only simple deicing systems, or none at all, fly more and more often under weather conditions favoring icing. For example, if ice forms on the aircraft on the ground, the pilot must decide whether icing is within permissible limits or whether the ice layer should be removed before starting. Therefore, determination of the thickness of the ice and the positive or negative growth rate of a layer of ice is crucial for general flight safety as an important criterion for assessing the potential danger posed by a developing and changing layer of ice. The thickness of the ice layer is also significant for optimal use of deicing agents.
WO 96/26430 discloses a method for determining the surface state of travel routes and a device for carrying out the method in which, by means of spectroanalytic determination of water and ice on a fixed road surface in four different wavelength ranges by reflection measurement through the ice layer, the formation of ice and frost can be detected as a function of the respective degree of crystallization and the thickness of the ice layer on the surface can be taken into account. However, the different spectral shape of the absorption of the dry roadway surface is not compensated in the predetermined four wavelength ranges, so that while conclusions may be drawn concerning the surface state, accurate determination of the thickness of the ice layer is not possible.
DE 195 06 550 discloses another method for spectrometric determination of water and ice on roads and an arrangement for implementing the method by which formation of ice or frost on fixed surfaces can be determined depending on the respective degree of crystallization; but there is no compensation of the spectral shape of the reflectivity of the road surface by interfering surfaces or coating and by external light sources (daylight), so that it is impossible to determine with exactitude the thickness of the ice layer and its growth rate.
Proceeding from the prior art mentioned above, the invention has the object of providing a method for dependably determining the thickness and growth rate of an ice layer in a very accurate manner over longer time periods and a device for implementing the method which is economical and reliable and which permits the exact thickness of an ice layer as well as its growth rate to be determined in a simple manner.
In order to meet this object, the invention proposes a method by which the radiation incident on a surface covered by an ice layer and/or disturbing layer is separated spectrally by an imaging holographic grating connected with a line receiver into enough wavelengths that a correction of the ice absorption is achieved by comparing the measured radiation with a stored reflection curve of an uncoated surface and by combining the comparison values and determining the peak area in the wavelength region of the ice absorption, and wherein the thickness of the ice layer and/or disturbing layer andxe2x80x94by means of the ice thickness values obtained in the measurement intervalsxe2x80x94the growth rate of the ice layer are determined and displayed.
Accordingly, the method according to the invention not only makes possible an accurate determination of the thickness of an ice layer and its growth rate, preferably on structural component parts of aircraft, but also makes it possible to detect the thickness of disturbing coverings, e.g., water, dirt and deicing agents. Further, the positive and negative growth rate of an ice layer is determined by comparison calculations from a sufficient number of determined ice thickness values, so that the ice layer thickness and its growth rate on the surface of structural component parts of aircraft can also be determined in advance for briefly changing weather conditions and climatic influences.
Due to the fact that the thickness of the ice layer and/or disturbing layer is determined on a surface to be monitored, this method can also be used not only to determine ice thickness but to detect dry layers of dirt, so that additional risk states caused by contamination of structural component parts can be eliminated by means of the method.
It is advantageously provided that the determined absorption curves can be evaluated by a simple algorithm by means of a microprocessor of the controlling and evaluating unit and a fast, accurate determination of the ice layer thickness and its growth rate is accordingly ensured using the existing technical resources in an aircraft.
The spectroanalytic measurement is preferably provided in wavelength ranges from approximately 850 to 1150 nm, wherein the light is separated spectrally into so many wavelength regions that the spectral dependency of disturbing coatings such as water, dirt and deicing agents and of different light sources such as daylight, twilight and artificial lighting can be detected and used for correcting the ice absorption.
Further, it is preferably provided that the measured values of an uncoated surface are stored in a controlling and evaluating unit as comparison values.
In addition, in order to increase measurement accuracy it is advantageously provided that the measured values from the long-wave end of the spectral region are removed from all measured values of a surface coated with an ice layer and/or disturbing layer because the light sensitivity, e.g., of a silicon receiver line, is negligibly small at that location.
Another advantage consists in that the ice thickness is determined in millimeters from the peak area which is proportional to the thickness of the ice layer by means of a conversion factor.
In addition, it is advantageously provided that the growth rate of the ice layer is determined from a large number of determined ice thickness values.
A particularly preferred further development consists in that the controlling and evaluating unit additionally issues a warning to clean the window or change the light source when the measured signal falls below a threshold in spite of the light source being switched on. In this way, additional sources of error can be eliminated from the determination of the thickness and growth rate of an ice layer.
A device for carrying out the method is preferably constructed as a spectrometer, wherein a window is provided in the monitored surface that is covered by an ice layer and/or disturbing layer, through which window a beam impinges on an imaging holographic grating via an optical imaging system and a source slit or entrance slit, is separated by wavelength, imaged on a line receiver and evaluated by means of a controlling and evaluating unit, and the evaluated data are displayed on a display.
In a preferred variant, the beam falling through the window in the surface covered with an ice layer and/or disturbing layer impinges directly on the holographic grating through the entrance slit.
In addition, it is preferably provided that a light source for illuminating the window that is provided in the surface covered by an ice layer and/or disturbing layer is arranged above the surface.
In an advantageous embodiment form, the light source for illuminating the window that is provided in the surface covered by an ice layer and/or disturbing layer is arranged below the surface.
In certain cases, a plurality of light sources which are arranged particularly annularly in a base body can also preferably be provided for illuminating the window that is provided with a surface covered by an ice layer and/or disturbing layer.
A further advantage consists in that, given sufficient daylight, the light source for illuminating the surface covered with an ice layer and/or disturbing layer is not turned on by the controlling and evaluating device.
In an advantageous further development of the invention, the arrangement of a semitransparent mirror in front of the optical imaging system results in an accurate and uniform illumination of the surface covered by an ice layer and/or disturbing layer.
Another preferred embodiment form consists in that the evaluating data can be sent to an existing data bus system, for example, in an aircraft.
Further, it is advantageous that the device is provided with an entrance slit, holographic grating, line receiver, and controlling and evaluating unit as a compact structural component part for all-purpose use.
The invention is directed to an apparatus for determining the thickness of a layer of ice on the surface of a stationary or moving object, for example, of a building or aircraft, in which
illumination light is initially directed to a portion of the surface of the object and then strikes a reception device,
the reception device contains dispersive optics for spectral dispersion of the illumination light coming from the surface portion and a detector for receiving the spectrally dispersed illumination light,
an evaluating device for determining intensity values of the illumination light influenced by the ice layer at different wavelengths xcex and for comparing these intensity values with reference values from a surface that is free of ice,
the reception device is arranged inside the object, and the surface of the object has, at the portion to which the illumination light is directed, a window that is transparent for the illumination light and through which the illumination light passes.
In a preferred construction of the invention, the reception device comprises an optical imaging system, an entrance slit, an imaging holographic grating and a silicon detector having individual reception elements. After passing through the window and being influenced by the ice layer, the illumination light is directed via the imaging system and entrance slit to the holographic grating, where it is spectrally dispersed.
The spectrally dispersed light strikes the silicon detector, each of whose reception elements receives an associated spectral component and an associated wavelength xcex and converts it into a reception signal which corresponds to the intensity of the light at this wavelength xcex. The reception elements are also commonly known in technical circles as pixels.
A computation circuit is provided in the evaluating device for processing the reception signals. The computation circuit is linked to at least one clock generator and an analog-to-digital converter. These components are advantageously combined in an integrated circuit.
The reception signals are read out of the silicon detector in the rhythm of a predetermined clock frequency, converted into digital values by the analog-to-digital converter and supplied to the computation circuit. The digitized values are processed in the computation circuit essentially as follows:
Net signals are calculated from the individual reception signals, while the reception signals of the pixels for which no light signal has been detected are ignored.
A spectral range xcexmin1 to xcexmax1 is selected from the total spectrum received by the detector and the values of the reception signals originating from this spectral range xcexmin1 to xcexmax1 are logarithmized.
The logarithmized net signals are then multiplied pixel by pixel by an evaluation function B(n) and summed corresponding to the following function: I1=xcexa3*B(n)*L(n). A value I0 determined in the same way as value I1 is subtracted from the result I1, but based on the reception signals that are registered when no ice layer is present on the surface.
Value I0 is, for example, determined once during the manufacture of the apparatus. It can also be updated occasionally, for example, in order to calibrate the apparatus.
The value I=I1xe2x88x92I0 corresponds to the thickness of the ice layer and is advantageously indicated on the display as a measurement number in units of mm.
Further, the evaluating device can be outfitted with a data storage for the values I1 determined at given time intervals and can comprise another computation circuit which compares the values I1 determined at different times with one another and determine information about the change in thickness of the ice layer over time from this comparison. The increase or decrease in thickness of the ice layer over a given time period can easily be determined from this change and can likewise be indicated on the display.
Daylight as well as light from an artificial light source may be used as illumination light. The artificial light source is arranged in the vicinity of the window for this purpose. The light source can be located outside the object or, alternatively, can also be accommodated inside the object, for example, inside the wing of an aircraft. In this case, the artificial light source is arranged in the vicinity of the window, so that the light radiated by the light source initially passes through the window and then reaches the reception device.
On the other hand, the reception device is always arranged in the interior of the object, so that the illumination light passes through the ice layer and window to reach the reception device when the light source is arranged externally. If the light source is positioned in the interior of the object, the light initially passes outward through the window to the ice layer, is reflected by the latter, and passes through the window again before reaching the reception device.
When the light source, like the reception device, is also arranged in the interior of the object, all components of the apparatus according to the invention are advantageously accommodated so as to be protected from climatic influences.
An infrared light source can be provided, for example, as artificial light source. However, other light sources can be used also and, as was stated above, it is also possible to use daylight of sufficient brightness for evaluation.
According to the invention, wavelengths xcex in the range of about 850 nm to 1150 nm are preferably used for evaluation. It is advantageous when the window is constructed as an optical filter which is transparent for light in the wavelength range from 850 nm to 1150 nm. In this way, all other spectra of no significance for the evaluation can be filtered out.
Another particular construction of the invention has a threshold switch which is coupled to a sensor for evaluation of the daylight brightness and which is connected to the artificial light source in such a way that the light source is switched on only when the brightness of the daylight is below a given threshold and is not sufficient for evaluation.
The evaluating device is connected to a display for indicating the measurement results.
In another option for displaying the measured values on the display, the evaluating device is coupled to an acoustic or optical warning device to notify of measurement results below a given limiting value.
The invention is not limited only to the observation and measurement of ice formations on the surface of the object under consideration. It can also be used in a technically equivalent manner to observe and measure other kinds of deposits on the surface capable of optical evaluation, such as deposited dust, moisture coating, deposits of deicing agents and the like.